Process Capability (Cp and Cpk) Study of CNC Machines

Abstract

The accuracy of a machine tool is generally assessed in accordance with the relevant BIS/ISO standards, which serves mainly to provide information concerning the geometrical and other accuracies, as a rule with the machine in unloaded condition. Besides these tests, there are other factors important while assessing the quality and acceptance of the machine tool, based on of its capability in machining, which is considered effective and useful to the seller and buyer of the machine. BIS/ISO-230 standard define the machine tool acceptance based on its testing for geometrical errors and other performance parameters, whereas ISO-26303, ISO-22514 and VDI-DGQ-3441 standards provide a procedure for the acceptance of a machine tool based on of its capability in machining. The statistical verification process described in these standards is applicable to all machine tool categories, and it is necessary to follow typical test pieces and machining conditions, as per guidelines indicated in the VDI-DGQ standard.ISO recommends to calculate Cp/Cs using standard deviation by normal method, whereas VDI-DGQ recommends use of Range method, and trend to be corrected to estimate for Cp, if observed. ISO standard defines Cpk, based on upper and lower tolerance limits, and also method for surface finish like errors, including the permissible values. This document intends to provide consolidated information how the Cp/Cpk is defined in ISO and also the method of calculation by an example. There is lot confusion in the minds of industry, to calculate the Cp/Cpk and the recommended permissible values for the machine performance. The purpose of this document is to make the industry people (Manufacturers and users) to understand the calculation of Cp, well talked by the people.The purpose of this study is to bring out the differences defined between these various standards, explain the method of estimating the process capability index with an example, and also provide source for the recommended permissible values as per relevant standards, to help industry partners in understanding and the calculation of the process capability index.